This invention relates to a process for producing zirconium sols and gels, and a process for producing zirconia using such sols and gels.
As disclosed in U.S. Pat. No. 3,423,193, it is already known that the reaction in water of zirconyl chloride and boric acid in the presence of hydrochloric acid produces, as water-soluble precipitates, boratozirconium chloride containing both boron and chlorine in amounts equimolar to zirconium, respectively, and having the formula (HBO.sub.3) (ZrOH)Cl.multidot.xH.sub.2 O. It is also disclosed that the boratozirconium chloride forms an acidic viscous sol solution when being dissolved in water, and the sol forms gels when being mixed with organic solvents such as methanol or ethanol. However, since the above reaction is carried out in very strong acidic conditions due to a strong acidity of zirconyl chloride, the reaction is unsuitable for industrial production of boratozirconium chloride sol.
On the other hand, the use of zirconium oxide or zirconia is now rapidly expanding. For example, pure zirconia is used as piezoelectric materials; stabilized zirconia is used as oxygen sensors; and partially stabilized zirconia is used as structural materials such as engine parts as well as industrial tools and blades.
As is well known, when being heated, zirconia changes its crystallographic form from monoclinic to tetragonal form at about 1000.degree. C., and changes from tetragonal to cubic form at about 1400.degree. C. When being cooled, there takes place the reverse transformation in turn, and the transformation of zirconia from tetragonal form to monoclinic is accompanied by a large expansion of volume. Therefore, stabilized or partially stabilized zirconia which contains such metals as Y, Mg or Ca as stabilizers so as to form thermally stable solid solutions is widely used for the production of sintered molds of zirconia. The stabilized zirconia has the tetragonal form over a wide range of temperatures, while the partially stabilized zirconia has the tetragonal form together with monoclinic.
Various methods are already known for the production of stabilized or partially stabilized zirconia, and the representative methods are powder-mixing method and coprecipitating method. In order to produce zirconia which contains yttrium therein by the powder-mixing method, for example, a powder of zirconium oxide and a powder of yttrium oxide are mixed together in a predetermined ratio, and the powder mixture is heated at a high temperature. This reaction is a solid reaction which needs high temperatures more than about 1300.degree. C., and thus the resultant zirconia is unavoidably deficient in uniformity of compositions. According to the coprecipitating method, an aqueous solution which contains both of a zirconium salt such as zirconyl chloride and an yttrium salt such as yttrium chloride, for example, is first prepared, and a common precipitant such as ammonia is added to the solution to coprecipitate zirconium hydroxide and yttrium hydroxide, followed by calcination of the coprecipitate at about 1000.degree. C., to provide stabilized or partially stabilized zirconia depending upon the amount of yttrium used. However, since the coprecipitate is not fixed in compositions, the resultant zirconia is also deficient in uniformity of composition.
A further method is also known in which alcoholates of zirconium and yttrium, for example, are concurrently hydrolyzed to form coprecipitates of hydroxides, followed by calcination of the coprecipitate, but this method also fails to provide zirconia having a fixed composition, similarly to the coprecipitating method.
As described as above, it is difficult to produce zirconia which is fixed and uniform in composition by the prior methods, and furthermore, all of the prior methods require complicated procedures resulting in high costs for the production of zirconia.